Reset allocation pools to [] instead of ''

the sdk expects subnet allocation pools to be reset to an empty array,
not an empty string.

Currently this results in an error message:

  "Invalid input for allocation_pools. Reason: Invalid data format
   for IP pool"

Change-Id: I7cc84b9c8e4abdbd2c91e5d591ad31f0849c1a83
Closes-Bug: #1634672

Author: ecbaldwin

openstackclient/network/v2/subnet.py

@@ -542,7 +542,7 @@ def take_action(self, parsed_args):
             if not parsed_args.no_allocation_pool:
                 attrs['allocation_pools'] += obj.allocation_pools
         elif parsed_args.no_allocation_pool:
-            attrs['allocation_pools'] = ''
+            attrs['allocation_pools'] = []
         if 'service_types' in attrs:
             attrs['service_types'] += obj.service_types
         client.update_subnet(obj, **attrs)